1. Field of Invention
The present invention relates to mobile operations that disturb the seabed or aquatic bed, in particular, trailer suction dredging but also more generally to other trenching, ploughing or moleing operations to lay seabed pipelines or cables.
Whereas the invention is described in terms of “seabed” it is equally applicable to any aquatic benthic environment including an area of seabed, riverbed or lake bed.
All the aforementioned operations involving seabed-disturbing apparatus result in the inadvertent destruction of a significant component of the aquatic/marine ecology living on the bed of the sea or other aquatic environment. This unwanted impact complicates and lengthens the licensing process for dredging applications and other seabed development applications requiring detailed assessments of the effects on aquatic/marine ecology and frequently licence conditions to mitigate the impact.
The present invention provides an apparatus and method that gathers benthic1 life and life supporting material from the seabed in front of the said seabed-disturbing-apparatus and relocates the said benthic material away from the immediate working area. 1 Benthis=the term given to bottom dwelling species whose natural habitat is near the bottom of a lake stream river or ocean.
The invention will be described first in relation to its application to trailer suction dredging. Its potential application to seabed trenching, ploughing and moleing operations will then be discussed.
2. Description of the Related Art
Suction dredging dates from the 1930s when deck mounted pumps were used on vessels, in conjunction with suction pipes to remove material from the sea-bed or waterways.
Trailer suction dredging is an operation in which a vessel is provided with a pipe that is used to suck material, e.g. gravel, sand or silt, from the seabed or bed of a waterway and to discharge it into the hold of the vessel. One such system is described in patent No. NL1009096C as a pipe consisting of telescopic sections. The relative vertical positions of the sections are adjusted as the water surface rises or falls. The compensation mechanism can be controlled from the ship or from the device on the lower end of the pipe. Various techniques or combinations of techniques can be used to achieve the alterations in pipe length, e.g. extra mechanical drivers, a float system, fixed sections connected via grommets or a flexible section.
Suction dredging may be used for one of several purposes including:                The permanent removal of material from the aquatic/marine environments for the purposes of construction or change in channel depth or width known as capital dredging;        The periodic removal of sediment from the aquatic/marine environments in order to maintain navigation for vessels known as maintenance dredging;        The extraction of sand & gravel from the aquatic/marine environments for use as building materials or in coastal/flood defence schemes known as aggregate dredging        
The efficiency of systems developed on this principle has improved markedly with modern aggregate suction dredgers being able to extract up to 2500 tonnes material per hour. Technological improvements have focussed on a number of areas that include:                Swell/heave compensator devices                    See for example patents U.S. Pat. No. 4,102,064 and JP8239088                        The efficiency of the suction arrangement                    See for example patents U.S. Pat. No. 2,961,782, U.S. Pat. No. 6,189,243 and U.S. 5,979,088                        Minimising the silt generated by suction dredging                    See for example patents U.S. Pat. No. 4,541,927 and JP2004003210                        Selectivity in the particle size of material entering the suction pipe                    See for example patent JP7238566                        The efficiency of the cutting head                    See for example patents: WO2004111354 and CN1488822                        
With the above-mentioned enhancements, the basic principle remains unchanged: i.e. use of a vessel provided with a pipe that is used to suck material, e.g. gravel, sand or silt, from the sea bed or bed of a waterway and to discharge it into the hold of the vessel.
3. Outline of the Problem
The seabed constitutes one of the most significant components of marine ecosystems, providing the habitat for approximately 98% species in comparison with 2% species that occupy the water column. Similar ratios apply to estuarine environments. In seabed environments, the benthic (seabed) fauna either lives on or attached to the seabed surface as epibenthic species (including crustacea such as crabs and lobsters, molluscs such as scallops and mussels as well as demersal fish species including plaice and sole) or as infauna occupying predominantly the top 0.5 m sediment. Many species of fish forage the seabed for food, and significantly certain species bury themselves in the sediment to provide cover from both prey and predators.
In the light of the above, one of the principal impacts of seabed disturbing operations and marine aggregate dredging, in particular, is the loss of benthic species from an area which has been dredged. Crustacea, fish and other organisms are sucked up with the targeted sand/gravel, leaving the seabed in the wake of the dredge largely devoid of organic material. The term organic used here is defined as material of, relating to, or derived from living organisms. Organic material whether in the form of biomass (live flora or fauna) or detrius (dead material derived from living organisms) is vital for recycling of nutrients to sustain ecosystems. Studies in relation to marine aggregate extraction have shown that a loss of up to 90% of biomass can result from dredging. In addition, a substantial proportion of the detrius such as shell debris will also be removed from the seabed/water bed during suction dredging.
Currently no method of suction dredging has been developed to reduce the organic material entering the draghead. A means of screening out a significant proportion of the organic material entering the draghead and returning it to the seabed would markedly reduce the impact of dredging on marine/aquatic ecosystems and accelerate the ecological recovery of benthic habitats. In relation to aggregate dredging, a method of operation which minimises the destruction to seabed ecology would accelerate the licensing process, reduce contamination within cargoes and so improve the quality of the aggregate resource being extracted.